Subsurface safety valves SSVs are safety devices mounted deep within wells to control flow to the surface. They generally have many components in common. The valve member is generally a flapper, which rotates 90° and is held open by a flow tube shiftable downwardly therethrough to cause the 90° rotation. This direction of movement (opening) is away from a closure or seat. A control system is generally employed to urge the flow tube in the opening direction involving hydraulic pressure from the surface connected to the SSV below via a hydraulic control line. In general, applied pressure opens the valve, while removal of applied pressure from the surface allows a power spring acting on the flow tube to move the flow tube in a direction opposite the opening direction and thereby out of the path of the flapper. This allows the flapper to pivot 90° to a closed position.
Various types of control systems have been employed for SSVs in order to address various different issues or interests of an operator. To reduce the size of the closure spring acting on the flow tube, reservoirs pressurized with a gas have been used to counteract the hydrostatic pressure from the column of hydraulic fluid in the control line that runs from the surface down to the SSV. Since the pressurized gas resists the hydrostatic force and offsets it, closure of the SSV is accomplished with a fairly small spring when the actuating piston, acting on the flow tube, is placed in hydraulic pressure balance, thus allowing the small closure spring to shift the flow tube and allow the flapper of the SSV to close.
Such systems include pressurized reservoirs having a gas on one side and hydraulic fluid (liquid) acting on the opposite side of an actuating piston. In order to make such systems work, numerous seals are used. Control systems have also been developed that serve to allow normal opening and closing of the SSV while, at the same time, restricting the valve to fail in a predesignated safe position in the event of an occurrence of any of a number of different possible conditions or events relating to component failures in the control system. U.S. Pat. No. 6,109,351 (hereinafter “'351” and which is incorporated herein by reference in its entirety), for example, describes such a control system.
With the large number of seals in such a system and the requirement that many of the seals must maintain a seal while statically engaging with a piston and slidably engaging with a cylinder bore; seals are a major source of such component failure. Though the failsafe control system prevents undesirable uphole flow when a seal failure does occur it remains a costly undertaking to withdraw the SSV from downhole to repair and/or replace the defective seal or seals and run the SSV downhole again. As such, the art will welcome seals that exhibit improved durability and reliability.